This invention relates to an apparatus for manufacturing composite structures. More particularly, the invention relates to an apparatus for manufacturing composite structures which are especially adapted for simplifying fabrication of a number of articles such as boats, ships, body parts for automobiles, trucks, trailers and the like.
U.S. Pat. No. 5,429,066 to Lewit et al. (hereinafter "Lewit et al.") discloses a composite structure and method of manufacturing same. Composite structures manufactured in accordance with Lewit et al. have met with substantial commercial success due to their superior structural characteristics and ability to simplify the fabrication of a number of articles such as boats and other reinforced plastic structures which are manufactured using similar techniques. Significantly, however, it has been found that the manufacture of composite structures in accordance with Lewit et al. can be time-consuming and therefore relatively expensive.
The composite structure disclosed in Lewit et al. is generally comprised of a structural foam core interior surrounded by an outer reinforcing fabric layer. A non-woven fabric layer, such as a mat fiber layer, is attached to the reinforcing fabric layer. A structural foam is attached to the non-woven fabric layer on the side of the non-woven fabric layer opposite the reinforcing fabric, by filling the interstices (pores) of the non-woven fabric layer.
Structural foams are commonly formed using two or more component parts which are mixed together immediately prior to the time that the foam is to be used. For example, the structural foam may be a two part, self expanding, self-curing urethane foam. The component parts are generally mixed together, either in a mixing fixture or in a container, prior to use. Subsequently the foam is deposited in a mold and allowed to cure. The component parts typically comprise a blowing agent which is combined with a resin.
One important factor which must be carefully monitored when manufacturing foam core composite structures is the mass ratio of component parts of the structural foam. If the mass ratio is incorrect, the structural integrity, stability, and water resistance characteristics will be undesirably altered. Due to variations in the consistency and viscosity of the constituent foam parts, it is often difficult to ensure consistent mixing of such parts in a proper mass ratio. In the case of composite structures requiring the injection of large amounts of foam in a mold, this does not create a substantial problem because the consistency and viscosity do not vary as much with high flow rates and are averaged out over time. However, where small amounts of foam are used, foam component ratio variations can create a serious problem.
In a continuous foam core production process as described herein, a second factor which must be carefully controlled is the total foam mass injected. If excessive amounts of foam are injected, the foam will have an undesirable tendency to expand through the non-woven fabric layers and into the reinforcing fabric layers when it is used for production of composite structures as described in Lewit et al.
A common type of structure which is fabricated using the techniques described in U.S. Pat. No. 5,429,066 to Lewit et al. (hereinafter "Lewit et al.") is an elongated beam or stringer (hereinafter "stringer") which may be formed with various cross-sectional profiles. Such stringers are commonly used as structural elements in boat construction and as component parts in many other larger fiber reinforced plastic structures which are manufactured using similar techniques. One method of manufacturing such elongated stringers involves use of elongated molds which can be lined with fabric layers as described above. The molds are then injected with structural foam which has been formed by mixing the proper ratio of constituent parts.
Due to the rather time-consuming process of forming stringers using elongated molds, it would be desirable to provide an apparatus capable of continuously producing a length of composite stringer, such as those which are described in Lewit et al. However, in order to manufacture a composite structure in this manner, careful control must be maintained over the instantaneous mass ratio of the component foam parts as well as the total instantaneous mass of foam injected. Particularly in those instances where the cross-sectional profile of the part defines a relatively small area, the rate of foam injection may be too low to ensure that any variations in the mass ratio of the constituent foam parts are averaged out over time.
Moreover, in the case of self-expanding foam of the type used in processes such as Lewit et al., at least one of the component foam parts is a blowing agent (such as nitrogen and HCFC's) combined with a resin, which must be maintained under pressure prior to use. The resulting component is a foamy, frothy mixture that is difficult to dispense accurately in terms of mass and volume. In fact, equipment of the prior art has generally been found to be capable of providing adequate control over foam component mass ratios only at flow rates above three pounds per minute when using pressurized foam.
Gear pumps such as those manufactured by Viking Pump, Inc. of Cedar Falls, Iowa, have been used to deliver a precisely controlled amount of component foam parts in those instances where the foam resin component parts are not pre-mixed with a blowing agent. However, it has been found that use of resin component foam material which has not been pre-mixed with a blowing agent can cause problems in the manufacture of products such as those described in Lewit et al. In particular, it has been found that where a blowing agent is not added to the resin foam component prior to mixing with the other foam component part(s), the combined component parts comprising the foam, which are expelled from a mixing nozzle, can soak through the layers of non-woven and reinforcing fabric layers lining a mold. This has the undesired result of occasionally allowing foam to penetrate through to the reinforcing fabric layer, which is preferably maintained free of such material in such applications.
Conversely, gear pumps have not been used in applications where pressurized foam components pre-mixed with a blowing agent are used, as such pumps have generally been perceived as redundant in a system where the materials are already under pressure and do not require a pump for causing a flow. Instead, adjustable valve or fixed orifice type flow controllers have been used to control the rate at which the pressurized foam component is allowed to escape. Significantly, however, such adjustable valve systems have been found to be ineffective at delivering a precisely controlled flow of foam component material which has been pre-mixed with a blowing agent, at flow rates of less than about three pounds per minute. Thus, there has arisen a problem with respect to controlling the flow rate of pressurized resin material which is pre-mixed with a blowing agent.
Further, the process of assembling a plurality of beams or stringers into a framework for manufacturing larger composite structures, such as boats and body parts for automobiles, trucks and the like has proven to be time consuming and expensive. One of the difficult and time consuming tasks associated with production of such products relates to the proper positioning of stringers for providing structural support. In manufacturing systems of the prior art, individual stringers are typically manufactured or cut to size, positioned by hand and finally laminated into place for providing any necessary structural support. Positioning jigs are also sometimes used for assisting in the positioning of the stringers. In either case, however, the process of locating the proper size stringer and ensuring that it is placed in the proper position, has proven to be a substantial source of labor expense in assembling these types of composite structures.
Accordingly, it would be desirable to provide an apparatus capable of continuously producing a composite structure of the type disclosed in Lewit et al. It would further be desirable to provide such an apparatus with a foam mixing and injection system capable of accurately dispensing a relatively small volume of foam, and which provides sufficient control over the mass ratio of the constituent foam parts, so as to ensure that the resultant structural foam suffers no defects. Finally, it would be desirable to provide a method and apparatus for manufacturing composite stringer elements in attached multiple sets (i.e., production of more than one beam using a single reinforced fabric layer) so as to simplify construction of larger reinforced composite structures using said stringers and reduce production times for an end user of such stringers.